1. Field of the Invention
The invention relates to a device and a method for detecting a magnetizable marker element, and specifically a magnetizable marker element of the type attached to an object for protection against unauthorized removal of the object. In particular, it is the purpose of the invention to allow the detection of the presence of a marker element with a known magnetic behavior or, in case of a marker element that may take two different kinds of magnetic behaviors, to allow the detection of the present state.
2. Description of the Prior Art
A great number of such systems, also called anti-theft devices, are known from prior art which are used to detect whether objects or articles have been removed without permission from a shop or a lending facility, such as a lending library for books, films, compact discs and the like. Generally, these systems are divided into two groups, namely systems in which responders respond to an electromagnetic interrogation field and include integrated electrical circuits, mostly oscillating circuits and their other corresponding circuitry, which are attached to the objects to be protected. Electric oscillations are caused in these responders through the interrogation field, the responders then transmitting signals which, upon detection, indicates that the responders are still attached to the object. The responders, mostly encapsulated in plastics tags or the like, have to be removed from the articles by the personnel so that no alarm is triggered. Examples of these systems that use radio frequencies are described in U.S. Pat. Nos. 3,838,409; 3,863,244; 3,961,322; 4,243,980; 4,281,321; 4,321,586; 4,642,640; and 5,051,727.
The second large group of systems relates to magnetizable marker elements that are magnetically reversed when passed through the interrogation or excitation field, which is an alternating magnetic field, whereby they influence the excitation field. Due to the hysteresis properties of the magnetizable marker elements, the magnetic flux density changes abruptly upon each magnetic reversal, which is detected by a receiving aerial or other receivers, possibly in the form of a conductor loop. The change in time of the magnet flux density in the receiver is thus composed of a change in time that is due to the change in the intensity of the magnetic excitation field, and of the abrupt change upon each magnetic reversal of the marker element. Examples of such systems are disclosed in the German Laid Open document 27 09 522 and U.S. Pat. Nos. 4,118,693; 4,153,183; 4,384,281; 4,527,152; 4,539,558; 4,622,542; 4,663,612; 4,686,154; 4,704,602; 4,791,412; and 4,888,579.
The systems using a magnetically reversible marker element include systems in which the marker element is removed from the article by the personnel so that no alarm is issued at the exit. However, systems are known in which the marker element is activatable and deactivatable, i.e., it may be sensitized or desensitized. Such systems are particularly employed in lending libraries where it is important to resensitize the magnetic element when the object is returned by the customer. The dual-status, activatable and deactivatable, marker element thus usually remains on the object. Such a surveillance system comprises a stationary surveillance unit for generating and detecting the excitation magnetic field at the exit of a shop (also referred to as "aisle"), as well as (desk) devices with which the marker element is activated (so-called resensitizers) or deactivated (so-called desensitizers) and with which the state (activated or deactivated) of the magnetic marker element may be checked (so-called verifiers).
A problem common to all known anti-theft systems is that they may trigger an alarm caused by an external interference influencing the magnetic interrogation and/or excitation field and not only when an object with an unremoved or not deactivated marker element is moved through the aisle. Erroneously triggered alarms are very inconvenient to the operator of the surveillance system since the customer will be suspected to have intended to take away the object concerned without permission.
A great number of methods for increasing the indifference to interference are implemented in the systems described in the above patent documents, according to which the output signal of the receiver device exposed to the interrogation field is processed and evaluated further in a certain manner. Systems are known, for example, which, by means of an amplitude detection, possibly with a previous filtering, determine whether the output signal from the receiver device has been caused by an interference or not (U.S. Pat. Nos. 4,118,693; 4,153,183; 4,704,602; and German Laid Open document 27 09 522). Further, methods are known in which correlation functions are calculated or certain harmonics of the received signal are determined, the amplitude values of which are evaluated and compared to preset values (U.S. Pat. Nos. 4,384,281; 4,527,152; 4,539,558; 4,622,542; 4,663,612; 4,791,412; and 4,888,579). Checking the received signal or certain harmonics of the received signal for exceeding predetermined upper or lower threshold values is a rather unreliable method that does not allow for sufficiently reliable results. Correlation function methods are generally rather unreliable, yet they require a comparatively complex circuit design with complicated signal processing.